Thermoelectric GNP-Integrated MoS2/S Cathodes for Mg-S Batteries with a Li2CO3-Modified Halogen-Free Electrolyte

Magnesium-sulfur (Mg-S) batteries are attractive energy-storage systems but remain limited by sluggish Mg2+ transport, interfacial instability, and polysulfide shuttling. Herein, a Li2CO3-modified halogen-free electrolyte based on Mg(NO3)2 in an acetonitrile/tetraethylene glycol dimethyl ether (ACN/G4) solvent system was combined with a MoS2/sulfur/graphene nanoplatelet (MoS2/S/GNP) composite cathode. The modified electrolyte exhibited an ionic conductivity of 1.4 × 10-4 S cm-1, representing a ∼17% increase relative to the pristine electrolyte, together with an enhanced Mg2+ transference number of 0.867. Symmetric Mg cells showed improved Mg plating/stripping stability and reduced polarization, indicating enhanced Mg/electrolyte interfacial compatibility. The MoS2/S/GNP cathode provided a conductive framework and catalytic sites for sulfur conversion. However, full-cell studies revealed substantial transport and interfacial limitations during cycling. The charge-transfer resistance increased from 237 to 2516 Ω after electrochemical activation, while the Mg2+ diffusion coefficient decreased from 1.00 × 10-14 to 1.26 × 10-17 cm2 s-1. Galvanostatic cycling showed rapid capacity fading from ∼ 310 mAh g-1 in the first cycle to near-zero values after ∼15 cycles, accompanied by Coulombic efficiencies exceeding 100% during the early cycles, indicating significant parasitic reactions and polysulfide shuttling. These results demonstrate that Li2CO3 modification influences Mg2+ transport and interfacial behavior but is insufficient to overcome the fundamental degradation processes limiting halogen-free Mg-S batteries. The study provides insight into electrolyte-cathode interactions and highlights key challenges for future Mg-S battery development.